Effects of an environmentally relevant temporal application scheme of low herbicide concentrations on larvae of two anuran species

Cultivation of herbicide-tolerant crops involves repeated applications of the complementary herbicide throughout the growing season, while in conventional corn production, herbicide application is restricted to the beginning of cultivation. Repeated application of herbicides increases both the likelihood an organism will be exposed to the herbicide and the concentration it may be exposed to. We examined effects of short and pulsed exposure of the cycloxydim-based herbicide formulation Focus® Ultra at doses close to the calculated LC5 (0.01 and 0.5 mg a.i. L−1) and LC10 values (0.05 and 1.0 mg a.i. L−1) on early premetamorphic and prometamorphice larvae of two anuran model organisms, Xenopus laevis and Discoglossus scovazzi. In addition, larvae were repeatedly exposed, i.e. at all considered developmental stages. The herbicide did not induce effects on body size at and time to metamorphosis or increase deformation rates in both species. Exposure to calculated LC5 values did not increase mortality or cause clinical signs in both species. At calculated LC10 values, narcotic effects were seen in all developmental stages. There was no clear evidence of developmental-specific mortality. Metamorphic success was independent of time point and duration of application in X. laevis. Only repeated exposure significantly increased mortality at metamorphosis in D. scovazzi. Narcosis may result in increased mortality under field conditions due to rise of predation risk. Different sensitivity of the test species to the compound was attributed to their physiological properties. Different filtering rates were understood as an accompanying factor influencing exposition.